The long term goal of this project is to generate libraries of novel polyketides for anticancer drug screening. Libraries will be constructed by manipulation of the gene cluster encoding the biosynthetic enzymes for the polyketide immunosuppressant, rapamycin. The rapamycin ployketide synthase (PKS) is an ideal candidate for such library development due to its large size (14 modules), modular layout, unusual starter unit and novel synthetic final step. A collection of mutant gene clusters will be made by varying the size and or the activities within each gene cluster ultimately using a combinatorial approach. The resulting engineered PKS gene clusters will be expressed in a specially constructed Streptomyces host/vector system. Polyketides are potent pharmacophores and it is anticipated that these libraries will prove extremely valuable as a source of lead compounds. Phase I of this project involves cloning the rapamycin PKS gene cluster from Streptomyces hygroscopicus and functional expression of a truncated gene cluster containing the first three modules in the heterologous Streptomyces host.